There was once a man in the state of Chu, who was selling shields and lances. He was praising them saying: “My shields are so firm, that there is nothing that can pierce them.” He praised his lances saying: “My lances are so sharp, that there is nothing that they cannot pierce.” Someone asked: “What if you used your lances to pierce your shields?” The man could not answer. A shield that cannot be pierced and a lance that can pierce everything cannot exist in the same world.